In building astronomical mirror telescopes, it must be ensured that the mirror retains its shape unchanged in every position. That is, the mirror must retain its shape independently of the inclination of its surface to the direction of the force of gravity. The usual requirements for the exactness of shape are a few 10 nm RMS.
A special support arrangement of the mirror is required for this purpose. This support arrangement prevents deformations of the mirror which are inevitably introduced by the carrier structure as a result of external forces and temperature fluctuations. For large telescopes, these deformations lie in the region of several hundred .mu.m and therefore exceed the desired exactness of shape by several orders of magnitude.
Heretofore, passive support systems were almost exclusively used for supporting such mirrors and for taking up the axial and radial mirror load. A known axial support system is described in the ZEISS-Information, Volume 27, Book 94 (1982), pages 8 and 9. In systems such as described in this publication, the support system distributes the support forces evenly through a sufficient number of individual support points on the backside of the mirror. The number and arrangement of the support points depend on the size, the total weight, and resistance to bending of the mirror.
For some time, there has been a tendency to make the mirrors ever thinner, yet their surface ever larger. In order to simultaneously satisfy the increasing requirements on the exactness of shape of the mirrors in spite of reduced resistance to bending, it is intended that the mirrors be forced into the correct shape by the purposeful application of external forces. For this purpose, it already has been proposed to use a so-called "active" supporting arrangement wherein the forces acting on the support points of the axial support system are computer-calculated and applied to the rearward side of the mirror by means of actuators whose forces are adjusted individually according to the computer-calculated distribution.
When the forces exerted by the actuators are correctly adjusted, the mirror can be purposely deformed in the sense of a restoration of the disturbed exactness of shape. In that case, residual errors due to the work done on the mirror surface also can be cancelled.
Such a support system is known, for example, from an article entitled "From Passive Support Systems to the NTT Active Support" by R. N. Wilson, F. Franza and L. Noethe which was published in the Proceedings of the IAU Colloquium No. 79: "Very large Telescopes, their Instrumentation and Programs", Garching, Federal Republic of Germany, Apr. 9 to 12, 1984, pages 23 to 40. The actuators proposed here consist of two essential elements: a sensor which measures the partial weight of the mirror acting on the individual support point and an electromagnetic arrangement in the form of a motor-driven lever system which applies the precalculated force. With the aid of a control arrangement, the motor is so adjusted that the force measured by the sensor corresponds to a calculated value which is composed of the passive base load and the additional force necessary for the deformation.
Since the partial amount of the support force to be controlled in this system is very small and only accounts for about 1% of the total force of a support point and since the sensor utilized for controlling the force measures the total force, very high requirements are imposed on the accuracy and time stability of the sensor which are not adequately satisfied. For this reason, it is difficult to actually apply the precalculated ideal force distribution onto the rearward side of the mirror. This difficulty is particularly introduced by the situation that an additional force purposely applied to a point requires a change of the forces at all the other points, which cannot self-adjust but can again be adjusted only by actively controlling all the other points.
German Pat. No. 35 21 973 discloses a support system for a telescope mirror which includes an active supporting arrangement wherein individually adjustable additional forces can be applied to the mirror at individual support points with the aid of actuators. In addition to this active support, the system provides for a passive support in the form of a hydraulic system wherein the load is branched. The actuators are linear motors which are connected in a force-tight manner to the supports of the passive support arrangement. The configuration of a support unit according to this known support system is shown in FIG. 1a of the drawings.
FIG. 1b of the drawings is a schematic showing the distribution of the support points of an axial support system on the rear side of a mirror. In FIG. 1b, reference numeral 15 identifies the mirror to be supported. The mirror body is supported by 27 supports with respect to its axial weight components of which eighteen are arranged at equal spacing on an outer circle and nine are arranged on an inner circle with a smaller diameter. The supports are part of a hydraulic astatic load relieving system with the pressure chambers corresponding to the supports being arranged into three groups. The pressure chambers of these groups are interconnected by lines 5 and 6. The groups are arranged in respective ones of three 120.degree. sectors A, B and C. As a consequence of this distribution, the hydraulic support is indifferent with respect to tilting of the mirror.
Each of the 27 support points is superposed with an additional force of a precalculated amount with the aid of control unit 18.
Referring to FIG. 1a, each support point includes a housing 1 which is partitioned by a membrane 2 into an upper chamber 3 and a lower chamber 4. The chambers are filled with a hydraulic fluid which for the purpose of pressure equalization is connected with the corresponding chambers of the other support points of the system by means of lines (5, 6). The membrane 2 carries a centered plate 7 which is connected to a push rod 8 and the plate 16 is attached to the latter for carrying the mirror. The plate 7 also includes a stub-like rod 9 formed thereon for transmitting the calculated additional force for the active mirror deformation. The rods (8, 9) are sealed with respect to the housing 1 with the aid of membranes (10, 11).
Two pressure systems separate from each other are formed because of the division into two chambers (3, 4). If the mirror 15 is always in the horizontal position, then only the lower chambers 4 would be required. Since the mirror, however, must be tilted, and in an inclined position the weight of the hydraulic fluid itself additionally acts on the support points, and indeed differently depending on the positions of the support points, the upper chambers 3 are provided and compensate for this additional pressure.
The lower extension of the pressure plate 7 formed as a stub-like rod 9 is connected to the permanent-magnet armature 12 of a linear motor. The armature 12 exerts a force in the direction of the rod 9 which is proportional to the current through the coil 13 which surrounds the armature 12 and is connected to the control unit 18. This force is transmitted directly through the stub-like rod 9, the plate 7, the rod 8, and the carrier plate 16 onto the rearward side of the mirror 15 and so is superposed on the supporting force of the plate 7 of the passive hydraulic support system.
The known support system is indeed self-adjusting with reference to the base load of the mirror and requires only an adjustment of the additional force necessary for the active support. However, this support system has the shortcoming that also in the stationary state, energy will be transferred from the linear motors to the support points. Localized heat sources arise in this way which can deform the mirror and the support system.